The Google App Engine home is here http://code.google.com/appengine/ and contains excellent documentation on how to get stated. I don't want to repeat the documentation, so I'll just point out the essentials relevant to this blog.
The Getting Started guide can be found here: http://code.google.com/appengine/docs/java/gettingstarted/ for Java, and a Python equivalent is available too.
This example is based on the previous blog, the Sieve of Eratosthenes as a simple prime number generator. The request accepts one parameter n whose integer value is the upper limit to generate all the primes until (from 2). The response output is JSON encoded to create a simple web service.
Starting with the very basics, the recommended project directory structure is of the form:
|-src
|---META-INF
|---primes
|-war
|---WEB-INF
|-----classes
|-------META-INF
|-------primes
|-----lib
For any Web App for the Google App Engine, one additional GAE specific file is required alongside the standard web.xml deployment descriptor. This file is called appengine-web.xml and contains the name of the Application, for example:
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
<application>primes-chillipower-com</application>
<version>1</version>
</appengine-web-app>
In this simple GAE deployment descriptor the main thing is that the name of the application is specified, in this example the application was called 'primes-chillipower-com'.
Applications must be created on the App Engine, this can be done here: http://appengine.google.com/
Note that there are some limitations currently, you are allowed up to a maximum of 10 applications and applications can not be deleted once created!
The simple Java version:
In the simple Java version we can place a simple Servlet in the src directory and build using ant.
Example Java servlet:
package primes;
import java.io.IOException;
import javax.servlet.http.*;
public class PrimesServletJ extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws IOException {
resp.setContentType("text/plain");
int n = 100;
String nStr = req.getParameter("n");
try {
n = Integer.parseInt(nStr);
}
catch (Exception e) {}
int[] primes = primes(n);
resp.getWriter().println("{ \"n\" : \"" + n + "\",");
resp.getWriter().println(" \"primes\" : [ ");
for (int i = 0; i < primes.length; i++) {
String sep = (i < primes.length - 1) ? ", " : "";
resp.getWriter().println(" { \"" + i + "\" : \"" + primes[i] + "\" }" + sep);
}
resp.getWriter().println(" ] }");
}
public static int[] primes(int n) {
// initially assume all integers are prime
boolean[] isPrime = new boolean[n + 1];
for (int i = 2; i <= n; i++) {
isPrime[i] = true;
}
// mark non-primes <= N using Sieve of Eratosthenes
for (int i = 2; i*i <= n; i++) {
// if i is prime, then mark multiples of i as nonprime
// suffices to consider mutiples i, i+1, ..., N/i
if (isPrime[i]) {
for (int j = i; i * j <= n; j++) {
isPrime[i * j] = false;
}
}
}
// count primes
int primeCount = 0;
for (int i = 2; i <= n; i++) {
if (isPrime[i]) primeCount++;
}
System.out.println("The number of primes <= " + n + " is " + primeCount);
int[] primes = new int[primeCount];
int idx = 0;
for (int i = 0; i <= n; i++) {
if (isPrime[i] == true) {
primes[idx] = i;
idx++;
}
}
return primes;
}
public static void main(String[] args) {
int n = 100;
if (args.length > 0 && args[0] != null) {
n = Integer.parseInt(args[0]);
}
int[] primes = primes(n);
for (int p : primes) {
System.out.println("Prime: " + p);
}
}
}
The ant build script goes in the root of the project, to start with I based mine on the example on the Google App Engine documentation here http://code.google.com/appengine/docs/java/tools/ant.html#Uploading_and_Other_AppCfg_Tasks
Using the standard ant script the project can be compiled and even run locally using the SDK.
ant compile
ant runserver
If you start the dev app server locally, it will be accessibly using a URL of the form:
http://localhost:8080/yourappcontextpath
Where the port 8080 is the default and can be changed in the script.
The App Engine SDK comes with appcfg.sh script in the bin directory, which can be used to perform operations such as uploading your app to the App Engine.
../../../../tools/appengine-java-sdk-1.2.1/bin/appcfg.sh update war
The Scala version:
The main tasks in converting this conventional Java Servlet based web app to a Scala based one is to:
- Rewrite the Java Servlet in Scala.
- Reconfigure the ant build.xml to modify the classpath to include the Scala libraries and to run the Scala scalac compiler to create the Java .class files from the .scala source files.
Since the Servlet API is a Java based API, there will of course be some reference to Java based classes in the Scala Servlet.
Example Scala based Servlet:
package primes
import javax.servlet.http._
class PrimesServlet extends HttpServlet {
override def doGet(request: HttpServletRequest, response: HttpServletResponse) = {
var n = 100
try {
val nStr = request.getParameter("n");
n = Integer.parseInt(nStr);
} catch {
case e : Exception => {
println("Exception e " + e.getMessage())
}
}
val ps = primes take n
val out = response.getWriter()
out.println("{ \"n\" : \"" + n + "\", \"primes\" : [")
var i = 0
for (p <- ps) {
val sep = if (i < ps.length - 1) ", " else ""
out.println(" { \"" + i + "\" : \"" + p + "\" }" + sep)
i = i + 1
}
out.println(" ] } ")
}
def primes = {
def sieve(is: Stream[Int]): Stream[Int] = {
val h = is.head
Stream.cons(h, sieve(is filter (_ % h > 0)))
}
sieve(Stream.from(2))
}
}
Example modified and build.xml file:
<project>
<property name="scala.home" location="/Users/chillipower_uk/dev/tools/scala/scala-2.7.5.final/" />
<property name="sdk.dir" location="/Users/chillipower_uk/dev/tools/appengine-java-sdk-1.2.1/" />
<import file="${sdk.dir}/config/user/ant-macros.xml" />
<path id="project.classpath">
<pathelement path="war/WEB-INF/classes" />
<fileset dir="war/WEB-INF/lib">
<include name="**/*.jar" />
</fileset>
<fileset dir="${sdk.dir}/lib">
<include name="shared/**/*.jar" />
</fileset>
</path>
<!-- Copy over Scala jars -->
<target name="init">
<property
name="scala-library.jar"
value="${scala.home}/lib/scala-library.jar"
/>
<path id="build.classpath">
<pathelement location="${scala-library.jar}" />
<pathelement location="${build.dir}" />
</path>
<taskdef resource="scala/tools/ant/antlib.xml">
<classpath>
<pathelement location="${scala.home}/lib/scala-compiler.jar" />
<pathelement location="${scala-library.jar}" />
</classpath>
</taskdef>
</target>
<!-- Copy Scala JARs -->
<target name="copyscala"
description="Copies the Scala JARs to the WAR.">
<copy
todir="war/WEB-INF/lib"
flatten="true">
<fileset dir="${scala.home}/lib">
<include name="**/scala-library.jar" />
</fileset>
</copy>
</target>
<!-- Copy App engine jars -->
<target name="copyjars"
description="Copies the App Engine JARs to the WAR.">
<copy
todir="war/WEB-INF/lib"
flatten="true">
<fileset dir="${sdk.dir}/lib/user">
<include name="**/*.jar" />
</fileset>
</copy>
</target>
<target name="compile" depends="copyscala, copyjars, init"
description="Compiles Java source and copies other source files to the WAR.">
<mkdir dir="war/WEB-INF/classes" />
<copy todir="war/WEB-INF/classes">
<fileset dir="src">
<exclude name="**/*.scala" />
<exclude name="**/*.java" />
</fileset>
</copy>
<scalac
srcdir="src"
destdir="war/WEB-INF/classes"
classpathref="project.classpath"
/>
<!--
<copy todir="war/WEB-INF/classes">
<fileset dir="src">
<exclude name="**/*.scala" />
<exclude name="**/*.java" />
</fileset>
</copy> -->
<javac
srcdir="src"
destdir="war/WEB-INF/classes"
classpathref="project.classpath"
debug="on" />
</target>
<target name="datanucleusenhance" depends="compile"
description="Performs JDO enhancement on compiled data classes.">
<enhance_war war="war" />
</target>
<target name="runserver" depends="datanucleusenhance"
description="Starts the development server.">
<dev_appserver war="war" />
</target>
<target name="update" depends="datanucleusenhance"
description="Uploads the application to App Engine.">
<appcfg action="update" war="war" />
</target>
<target name="update_indexes" depends="datanucleusenhance"
description="Uploads just the datastore index configuration to App Engine.">
<appcfg action="update_indexes" war="war" />
</target>
<target name="rollback" depends="datanucleusenhance"
description="Rolls back an interrupted application update.">
<appcfg action="rollback" war="war" />
</target>
<target name="request_logs"
description="Downloads log data from App Engine for the application.">
<appcfg action="request_logs" war="war">
<options>
<arg value="--num_days=5"/>
</options>
<args>
<arg value="logs.txt"/>
</args>
</appcfg>
</target>
</project>
Example request and output:
Request: http://primes-chillipower-com.appspot.com/primes?n=50
{ "n" : "50", "primes" : [ { "0" : "2" }, { "1" : "3" }, { "2" : "5" }, { "3" : "7" }, { "4" : "11" }, { "5" : "13" }, { "6" : "17" }, { "7" : "19" }, { "8" : "23" }, { "9" : "29" }, { "10" : "31" }, { "11" : "37" }, { "12" : "41" }, { "13" : "43" }, { "14" : "47" }, { "15" : "53" }, { "16" : "59" }, { "17" : "61" }, { "18" : "67" }, { "19" : "71" }, { "20" : "73" }, { "21" : "79" }, { "22" : "83" }, { "23" : "89" }, { "24" : "97" }, { "25" : "101" }, { "26" : "103" }, { "27" : "107" }, { "28" : "109" }, { "29" : "113" }, { "30" : "127" }, { "31" : "131" }, { "32" : "137" }, { "33" : "139" }, { "34" : "149" }, { "35" : "151" }, { "36" : "157" }, { "37" : "163" }, { "38" : "167" }, { "39" : "173" }, { "40" : "179" }, { "41" : "181" }, { "42" : "191" }, { "43" : "193" }, { "44" : "197" }, { "45" : "199" }, { "46" : "211" }, { "47" : "223" }, { "48" : "227" }, { "49" : "229" } ] }
The iPhone client:
I'm currently developing using the iPhone 3.1 beta SDK. I'm not going to post all the source code the the simple client, but rather give some essential snippets relevant to the invocation of the simple JSON web service once deployed on the Google App Engine.
To Consume this JSON in an iPhone app, we can use the JSON Objective-C library, which is available on Google code here http://code.google.com/p/json-framework/wiki/FAQ
- (void)loadData {
[super viewDidLoad];
NSString *urlString = [NSString stringWithFormat:@"http://primes-chillipower-com.appspot.com/primes?n=1000"];
NSURL *url = [NSURL URLWithString:urlString];
NSDictionary *primesResponse = (NSDictionary *)[JsonClientHelper fetchJSONValueForURL:url];
if (primesResponse != nil) {
NSArray *primeEntries = [primesResponse objectForKey:@"primes"];
NSMutableArray *primesTemp = [[NSMutableArray alloc] init];
[primesTemp retain];
for (int i = 0; i < [primeEntries count]; i++) {
NSDictionary *primeEntry = (NSDictionary*)[primeEntries objectAtIndex:i];
NSString *key = [NSString stringWithFormat:@"%d", i];
NSString *p = [primeEntry objectForKey:key];
NSString *msg = [NSString stringWithFormat:@"Prime %d = %@", i, p];
NSLog(msg);
[primesTemp addObject:p];
}
primes = [primesTemp copy];
[primesTemp release];
}
}
The above code makes a request to the JSON web service on the App Engine and decodes the response, which once decoded using the JSON library is contained in Dictionaries (NSDictionary) and Arrays (NSArray).
Where JsonClientHelper contains a basic class helper method fetchJSONValueForURL to fetch and decode a JSON response from the supplied URL.
+ (id)fetchJSONValueForURL:(NSURL *)url
{
NSString *jsonString = [[NSString alloc] initWithContentsOfURL:url encoding:NSUTF8StringEncoding error:nil];
id jsonValue = [jsonString JSONValue];
[jsonString release];
return jsonValue;
}
I hope this Blog has helped give some insight into creating a Scala based Web Application for the Google App Engine, as well as the consumption of JSON in an iPhone Objective-C client.
(I appologise for the formatting of the code in this blog, finding it awkward to get the code into the blog without it being mangled in some way!).
Resources:
Handy JSON Lint tool: http://www.jsonlint.com/
Scala Servlet development: http://blogs.oracle.com/aseembajaj/2008/08/scala_servlet_development.html
Scala on the Google App Engine: http://froth-and-java.blogspot.com/2009/04/scala-on-google-appengine.html
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